This invention relates to a liquid crystal display device, particularly to a liquid crystal display device of the twisted nematic type (hereinafter sometimes referred to also as "TN type") which is suitable for a multiplex driving system.
Liquid crystal display devices find applications primarily of the TN type in uses which are now expanding for having such advantages that power consumption is small, that production cost is low, that it can be made lightweight and thin, that it is easily applicable for color display, etc.
A liquid display device of the TN type is constituted by sealing a nematic liquid crystal having a positive dielectric constant anisotropy between two sheets of electrode substrates subjected to orientation treatment, and is generally made under the state in which the liquid crystal molecules are sequentially twisted by 90.degree..
The liquid crystal display device of this type with a twisted angle of 90.degree. (hereinafter sometimes called also as "90.degree. twisted type") can be produced simply and is suitable for mass production, and also has the advantage of rapid response.
However, the liquid crystal display device of the 90.degree. twisted type has moderate intensity change in the transmitted light (or reflected light) to the change in applied voltage and therefore has such problems as low contrast with sharp images, and also the angle of field of vision is narrow when the time sharing order is made greater in multiplex driving, thus being limited in application for high multiplex driving systems. For example, in a liquid crystal display device with a size of the display picture face of about A4 size, the duty ratio in multiplex driving has been practically preferred to be 1/200 or higher, but said duty ratio in the liquid crystal display device practically applied is about 1/100, with its contrast ratio (ratio of luminance during selection to that during non-selection) being as low as about 3.
As a technique for solving the problems of such a 90.degree. twisted type liquid crystal display device, Japanese Provisional Patent Publication No. 107020/1985 discloses a liquid crystal display device characterized in that the twisted angles of liquid crystal molecules are 180.degree. to 360.degree. and the angle between the director direction of the liquid crystal molecules oriented on at least one of the electrode substrates and the electrode substrates surface (hereinafter sometimes referred to also as "pretilted angle") is greater than 5.degree.. According to this liquid crystal display device, since the intensity change of the transmitted light to the applied voltage is steep, it is stated that a high contrast ratio of 19.6 can be realized in the case of multiplex driving at a duty ratio of, for example, 1/100.
However, in this liquid crystal display device, no sufficient consideration has been paid to the bistable effect and, for this reason, there is the problem that response is slow when the liquid crystal display device is subjected to multiplex driving at a high duty ratio. More specifically, in the liquid crystal cell of this liquid crystal display device, there occurs ordinarily the so-called hysteresis phenomenon in which the intensity change of the transmitted light is different during increasing the applied voltage than that during decreasing of the applied voltage. Due to the bistable effect on account of this phenomenon, there are involved the problems such that the practical voltage range during time sharing driving may be narrowed or the response time for on-off may be elongated. Also as the result of these problems, poor display quality is liable to occur by non-uniformity of the liquid crystal cell thickness, the temperature change, etc. For this reason, it becomes necessary to suppress the bistable effect.